The invention concerns a method, a pressing tool, and a punch, a pressing tool according to the categorizing portion of claim 4, and a punch according to the categorizing portion of claim 11. Connecting structural parts to a plate in accordance with this type of method involves non-detachable mounting of additional structural parts to a plate consisting preferably of relatively thin sheet metal, but even of plastic material or other deformable and crushable materials. The structural parts may be elements projecting from the plane of the sheet metal, such as nuts, threaded bolts, hook couplings and the like, or other plates which are to be connected to said plate. An important element of this connection is the xe2x80x9csunken punchxe2x80x9d which remains in the plate after connection as a sort of riveting work piece.
In a known method, tool, or punch (DE-Patent 30 03 908 U.S. Pat. No. 4,459,073), a stud bolt comprising both a thread and a punch is pressed, using a punching and riveting procedure, with the punch into a plate, wherein surface parts are punched through an opening of a female mould located opposite to the punch, and the edge areas of the punch openings are displaced. The punch is hollow and its cutting edge punches a corresponding plate part, while surface parts of the plate located outside of this punching-out area are flanged by the female mould. An opposing punch is disposed in the female mould, comprising a central spreading body through which the cutting ring of the punch is forced apart, i.e. widened in a further step thereby engaging behind the collar of the deep-drawn hole of the plate. Irrespective of the fact that the required female mould tool, comprising a spreading body, is technically demanding and is subjected to considerable wear, there is the disadvantage that the punch must consist of relatively soft material to be widened after punching the plate, and to subsequently be able to flow, in the female mould opening, below the deep-drawn edge. In accordance with the punch part, the threaded bolt part of this stud bolt is also of relatively soft material with all associated disadvantages. One main disadvantage is that, to effect mounting, the plate has to be penetrated thereby not only resulting in loss of rigidity but also generating non-sealed locations. The punch material must have a minimum rigidity in order to carry out the punching process which is contradictory to the material softness required for forming.
Another conventional method and tool categorizing the invention (WO 93/10925 A) comprise a sunken punch having a inwardly directed bulge at its side facing the plate, the bulge forming a cutting edge to effect flow of the sunken punch during the deep drawing process. The material constituting the sunken punch is sufficiently soft that the plate is not broken through. Depending on material hardness, a connection point of this kind is relatively malleable such that only unsatisfactory fulfillment of a major advantage of the sunken punchxe2x80x94namely, prevention of a pressxe2x80x94button character of such a connection pointxe2x80x94is effected.
In contrast thereto, the method, pressing device or punch in accordance with the invention having the features of the independent claims have the advantage that a highly rigid connection between plate and structural parts is produced with neither leakage nor weaknesses due to openings in a plate. Through deep-drawing, the plate material is sectionally thinned and moved in the deep-drawing direction in response to approximately 20% of the overall necessary force. The remaining 80% is increasingly required for the subsequent crushing process between the end face of the punch and bottom of the female mould opening, wherein the deep-drawn material first flows in a radially outward direction and then, due to the unyielding walls of the female mould opening, towards the punch thereby surrounding same in a form-fitting, rivet-like manner. Deformation of the punch is possible at the end of this crushing process due to the associated larger applied forces. All this is carried out in only one step. It is decisive that the punch serves as deep-drawing tool and also as xe2x80x9csunken, lost, or dead rivetxe2x80x9d. In order to ensure uniform quality, the material of the punch must be significantly harder than the plate material such that either only the plate material flows while the punch shape remains unchanged, or that the flowing process starts only near the end of the crushing process when the forces are large. In any case, a uniform quality is guaranteed for the individual connecting points produced according to the method.
The pressing device in accordance with the invention most advantageously elevates the edge region of the bottom of the deep draw opening. In contrast thereto, conventional pressing devices (DE Publication 4404 659) have a deep-drawing opening whose bottom has a peripheral depression. Material is thereby pushed into this depression during the pressing procedure thereby producing a push-button-like hook engagement with corresponding material accumulation serving to strengthen the hook engagement. In contrast thereto and in accordance with the invention, the peripheral elevation causes material to be displaced precisely in this area from the bottom of the deep-drawn, flat parts of the plate in a radial direction about the punch towards the top, thereby improving the hook engagement, and most importantly, effecting a rigid form-fit connection to the sunken punch which, in accordance with the invention, remains in the penetration opening.
In accordance with a further advantageous embodiment of the inventive method (per se known through DE Publication 35 32 900) the female mould is pot shaped and has a non-changing opening volume in the pressing direction and transverse thereto, to unyieldingly restrict the crushing process of the displaced material in the longitudinal and transverse directions. Appropriate design of the female mould opening to effect favorable material flow can thus achieve particularly high rigidity for the connection. In a method categorizing the invention (WO 93/10925 A), the female mould has flexible walls forming a collar and only the bottom of the mould is inflexible. The flexibility of the walls substantially influences the quality of the connection point, since insufficient flow of plate material occurs. However, the female mould can be fashioned in various ways; in further variations, e.g. having rigid cylindrical side walls or in the form of a pot having an adjustable bottom.
In an advantageous embodiment of the pressing device in accordance with the invention, the deep draw opening is a cylindrical bottom hole or tapers slightly in the pressing direction, with non-yielding side walls.
In accordance with an additional advantageous embodiment of the invention a sunken punch is provided which is part of the pressing plunger and is disposed between the plunger and the plate for deep-drawing the plate into the deep draw opening of the female mould for integration at this location.
In accordance with a further associated embodiment of the invention, the peripheral elevation has the same or a larger inner diameter than the punch diameter thereby preventing production of a bottle neck between the upper side of the elevation and the lower side of the punch which would impair material flow.
In accordance with a further advantageous embodiment of the invention, the press plunger comprises a central blind bore in its end for guiding the sunken punch. Such a central blind bore is known per se (DE Patent 30 03 908), wherein during fitting of the sunken punch, the press plunger can be supported on a shoulder or end face of the punch and a threaded bolt or the like can project into the blind bore.
In accordance with a further advantageous embodiment of the invention, the press plunger is guided in a clamping member which comprises supply openings for the sunken punch (per se known: DE Publication 44 04 659) to load a further sunken punch during the return stroke and before a renewed pressing stroke in preparation for a new connection point. This clamping member can simultaneously clamp the plate on the female mould before pressing the press plunger and punch into the plate surface. Automatic loading of the tool for the sunken punch can be effected via such a supply opening.
In accordance with a further advantageous embodiment of the invention, the plunger stroke is limited such that separation of the plate is avoided and the deep-drawn planar parts are still crushed. Such stroke definition is no problem for a pressing tool.
In accordance with an embodiment, the punch in accordance with the invention comprises one radial recess into which material can be displaced and engage during crushing, which can be formed as an annular groove or peripheral recess.
In accordance with a further embodiment of the invention of this type, two parallel annular grooves are provided in the punch having a minimum separation to avoid its breaking during crushing. In particular the high pressure required for the crushing process generates separating forces within the annular groove due to the displaced plate material which could lead to tearing off. The resulting xe2x80x9cbreaking pointxe2x80x9d is matched to the punch material and to the female mould shape, wherein the annular grooves may also be omitted in the blank to achieve such a xe2x80x9cbreaking pointxe2x80x9d.
In accordance with a further embodiment of the invention of this type, the transitions on the radial surface area and/or on the end face of the punch facing the pressing direction are rounded. Such rounding of the transitions improves flow of the displaced, crushed material.
In accordance with a further advantageous embodiment of the invention, the punch serves as connecting means with other structural parts and is provided with a corresponding connection engagement means. Such an engagement means may be either a pin projecting beyond the plate after installation on which an outer thread, an engaging annular groove or the like may be disposed, or a threaded bore, hook groove or the like on the punch or magnetic or other engagement means. In accordance with the invention, in addition to a connecting means comprising such an engagement possibility, further plates may also be connected to the plate as described above. It is decisive that the punch remains as a sunken tool in the pressing opening.
In accordance with a further advantageous embodiment of the invention, the radial surface area of the punch is formed as a rotation-preventing prism such that the displaced material prevents rotation of the punch in the opening receiving same.
An embodiment of the inventive method and of the punch is shown in a highly simplified manner in the drawing and further explained below.